1. Field of the Invention
The invention relates to a hybrid vehicle provided with an internal combustion engine and an electric motor as power sources, in which front wheels and rear wheels are driven independently from each other by the electric motor, and a control method of that hybrid vehicle. More particularly, the invention relates to a switching control apparatus which switches between two-wheel drive in which either the front wheels or the rear wheels are driven, and four-wheel drive in which both the front wheels and the rear wheels are driven, as well as a control method thereof.
2. Description of the Related Art
Some hybrid vehicles provided with an internal combustion engine and an electric motor that can also generate electricity (hereinafter referred as “electric motor”) as power sources are structured such that the front wheels and the rear wheels are driven independently from each other using the electric motor. For example, one known hybrid vehicle is structured such that the front wheels are driven by the internal combustion engine and the electric motor and the rear wheels are driven by another electric motor other than one that drives the front wheels. In such a hybrid vehicle, two driving methods can easily be realized simply by switching between operating and not operating one of the electric motors from among the electric motor provided for the front wheels and the electric motor provided for the rear wheels. These driving methods are two-wheel drive in which either only the front wheels or only the rear wheels are driven, and four-wheel drive in which both the front wheels and the rear wheels are driven.
Which of these two driving methods is to be used depends on control variables relating to vehicle running, such as vehicle speed, the gradient of the road on which the vehicle is running, the driving force required by the vehicle, vehicle steering angle, and the difference in rotation speed between the front and rear wheels. Four-wheel drive is used when a large driving force is required such as when taking off from a standstill, accelerating, or climbing a hill, or when running on a road with a low coefficient of friction such as an icy road. By distributing the driving force required by the vehicle between all four wheels, driving force is able to be reliably transmitted between each wheel and the road surface. On the other hand, two-wheel drive is generally used when not much driving force is required by the vehicle and the vehicle is driven on roads with a sufficiently high coefficient of friction. Running in two-wheel drive consumes less fuel and power than running in four-wheel drive as long as the wheels do not slip. With a hybrid vehicle such as that described above, two-wheel drive only requires that electric power be supplied to the electric motor for only one set of wheels, i.e., only either the electric motor for the front wheels or the electric motor for the rear wheels. Therefore, the electrical loss between the secondary battery and the electric motor is low, thereby enabling power consumption to be reduced when driving the vehicle.
Also, hybrid vehicles realize various driving modes by using the internal combustion engine and the electric motor selectively or in combination. Some examples of these driving modes include an “HV driving mode” which uses the internal combustion engine and the electric motor together as the driving source, an “engine driving mode” which uses only the internal combustion engine as the driving source, and an “EV driving mode” which uses only the electric motor as the driving source. These driving modes can be automatically switched successively depending on the state-of-charge (hereinafter simply referred to as “SOC”) of the secondary battery that supplies electric power to the electric motor, and the required driving force and the like.
Some hybrid vehicles having these kinds of driving modes are provided with an “EV switch” in the vehicle cabin to enable an occupant to intentionally select the “EV driving mode”. When the EV switch is turned on, the vehicle is placed in the EV driving mode. For example, international publication No. 00/05094 pamphlet proposes providing a lever in the vehicle which enables an occupant to switch from driving using the internal combustion engine as the driving source (i.e., engine driving mode and HV driving mode) to the EV driving mode. The occupant forcibly places the vehicle in the EV driving mode by operating this EV switch or lever. As a result, vehicle is able to run quietly because it is running in the EV driving mode, i.e., without using the internal combustion engine. Accordingly, this EV switch is useful at times such as when driving through residential districts at night.
When the hybrid vehicle is running in the EV driving mode, i.e., running using only the electric motor as the power source, the electric power supplied from the secondary battery to the electric motor becomes the power that drives the vehicle. Therefore, the cruising distance of the vehicle when driving in the EV driving mode is determined by the SOC of the secondary battery. When the SOC of the secondary battery decreases from continuously driving in the EV driving mode, the internal combustion engine must be operated to charge the secondary battery. At this time, the EV driving mode is automatically cancelled by a control apparatus of the vehicle. That is, the EV switch is turned off and the internal combustion engine is started.
In this way, even if the occupant selects the EV driving mode with the EV switch, the EV driving mode may soon be cancelled due to a low SOC of the secondary battery, which is problematic in terms of merchantability. To counteract this, it is possible to increase the cruising distance in the EV driving mode by increasing the size of the secondary battery but this would increase the weight of the secondary battery and the space required to mount the secondary battery. Therefore, when the driver operates the EV switch and switches the driving mode to the EV driving mode, it is desirable to reduce the power consumption of the electric motor as much as possible in order to increase the cruising distance in the EV driving mode.
In a hybrid vehicle which is able to run while switching successively between four-wheel drive and two-wheel drive such as that described above as well, the condition for switching between two and four wheel drive, i.e., the region in which the vehicle operates in four-wheel drive, is set appropriately beforehand by testing corresponding to control variables relating to vehicle running. The region in which the vehicle operates in four-wheel drive is normally set to emphasize power performance and does not consider reducing power consumption when driving in the EV driving mode that was selected by the occupant operating the EV switch. Therefore, there is a demand for technology that enables power consumption to be reduced in the EV driving mode that was selected with the EV switch while satisfying the power performance during normal driving.